Conventionally, a HUD device is known, which is housed in a housing part of a mobile unit, and projects a picture on a projection surface of the mobile unit through an opening of the housing part. The HUD device displays a virtual image of the picture to be confirmed from an interior of the mobile unit. Patent Literature 1 discloses a HUD device including a polarizing plate as a first light polarizer bonded to a front surface of a liquid crystal layer of a liquid-crystal display by transmission illumination, and a polarizing plate as a second light polarizer bonded to a rear surface of the liquid crystal layer. Light source light emitted from the liquid-crystal display is a linearly polarized light. The light source light is ejected from a dustproof sheet made of translucent resin such as acrylics arranged at the opening of the housing part.